1. Field of the Invention
The present invention relates generally to a method of controlling the peak power demand in an electrical power distribution network by controlling the peak power consumption of individual loads such as air conditioning, or heating loads and, more particularly, to a method and apparatus for controlling the heating or cooling plants of individual space conditioning apparatus in a predetermined manner based on external commands.
2. Description of the Prior Art
One of the most serious problems confronting electric utility companies today is the great variance in total electrical demand on a network between peak and off-peak times during the day. The so-called peak demand periods or load shedding intervals are periods of very high demand on the power generating equipment where load shedding may be necessary to maintain proper service to the network. These occur, for example, during hot summer days occasioned by the widespread simultaneous usage of electric air conditioning devices. Typically the load shedding interval may last many hours and normally occurs during the hottest part of the day such as between the hours of noon and 6:00 p.m. Peaks may also occur during the coldest winter months in areas where the usage of electrical heating equipment is prevalent. In the past, in order to accommodate the very high peak demands, electric utility companies have been forced to spend tremendous amounts of money either in investing in additional power generating capacity and equipment or in buying so-called "peak" power from other utilities which have made such investments.
More recently, electric utility companies have turned to load shedding as a means of controlling peak demand and this has led to the use of the term "load shedding interval" to define the period in which the network load is controlled. It is desirable that a load shedding device limit power demand uniformly over the entire load shedding interval because the actual peak of power demand on the total utility grid could occur at any time during the load shedding interval.
In the prior art, several basic strategies and devices have been utilized for load shedding in order to limit the peak power demand on the power generating capacity of electric utility companies. One such mode involves sending signals either over the power lines or by utilizing a radio-type signal emanating from the utility to disconnect or interrupt the use of certain selected electric loads such as air conditioning compressors when the demand has reached a certain point. While this type of direct control of power consumption by the utility achieves usage cutbacks during peak periods which prevent the power network from becoming overloaded, in many cases, the great inconvenience to the user who may find his power disconnected for an inordinately long time may well outweigh the benefits of the load shedding.
An alternate method of control employed by utility companies to reduce peak power consumption on given networks involves the concept of duty cycling. This involves a time sharing over the network of certain amounts of the power during peak periods such that service is interrupted to selected devices on a time sharing basis. Thus, for example, on a ten minute per one-half hour duty cycle, all of the devices for which service is to be interrupt have their service interrupted ten minutes out of each one-half hour on a rotating basis which each ten minutes involving one-third of the device population. While this method of duty cycling does accomplish some load shedding, it has several disadvantages.
First, fixed-period duty cycling tends to destroy "natural diversity." Natural diversity may be illustrated in terms of many machines supplied by a common power network. A large group of air conditioning or heating machines which continually cycle ON and OFF to maintain comfort conditions in corresponding spaces have a natural tendency to operate such that the cycling pattern of each machine is in random phase with the cycling pattern of all other such machines in the power network. In this fashion, there is but a random likelihood that all of the air conditioning compressors or heating machines will be operating at the same instant. The tendency for this random operation is then called natural diversity. Any load shedding strategy which tends to synchronize the running periods of all the compressors or heaters in the utility service network reduces natural diversity. Synchronization causes significant spikes in power demand during the ON cycle of these devices and negates much of the benefits of the load shedding. If the devices to be interrupted are electric air conditioning and cooling units, for example, the chances are that all such units whose power supply has been interrupted will be calling for power at the end of the OFF cycle such that a spike in power demand will occur upon switching of the interrupted units at the end of each cycle.
Also, this method of load shedding may be defeated or overcome by the customer by the installation of an oversized air conditioning or heating unit such that it may maintain the temperature of the environment utilizing only that portion of time allotted to it. The net effect, of course, is that no real power is shed.
The general problems associated with all such prior art methods and devices is that while they may accomplish a certain amount of load shedding which benefits the electric utility, they largely ignore a very important factor-the impact of one or more modes of interrupted services on the customer or user. Abrupt or large changes in the environmental temperature of a conditional space are very undesirable from the standpoint of the customer.
Other prior art methods of load shedding include the timed resetting of thermostats to a higher setting in the summer during the air-conditioning season and to a lower setpoint during the heating season for a specified period or number of hours during the peak demand part of the day. This step change does result in a significant energy savings over a long period but yields only a relatively small power reduction at the peak load time. It represents an abrupt change in the temperature of the environment which is sensed by the inhabitants who are required to endure uncomfortable temperatures for this lengthy period of time. What has long been needed is a device which can achieve the required network load shedding with a minimum impact on occupants of the conditioned space.